Always machmir –
Don’t you see a hint when it comes knocking on your door? She obviously wants to study with you (ride home, yeah.. right!)
OK, seriously, if your’re in a co-ed college, and she is frum, (jewish is not enough in this case) then its ok.
If either of you are married, then its still OK, as long as the married person calls the spouse from the car.
Otherwise, this is a chesed that can go sour in a hurry.